1. Field of the Invention
The present invention relates to integrated semiconductor lasers.
2. Description of the Related Art
High-power, single-mode laser diode sources are employed in a wide variety of applications, such as medical sensing devices and high-speed, optical telecommunication network components. For example, Raman amplifier components for optical networks typically require pump powers of 1 Watt and above.
A variety of laser diode structures exist in the prior art, including edge-emitting stripe, tapered stripe, broad stripe, grating-surface-emitting, master oscillator power amplifier, grating-stabilized broad stripe, surface-emitting distributed feedback, and antiguided array structures. Important features of any given laser diode structure include: high output power, single near-Gaussian spatial mode, short- and long-term lasing mode stability over time, long-term reliability, compact packaging, manufacturability, and ease of coupling to an external waveguide such as an optical fiber. However, no single laser diode structure of the prior art adequately possesses all of these features.
Obtaining relatively high output power with single-mode emission by a laser diode is difficult. Structures having greater output power tend to produce either (a) multi-mode emission or (b) low long-term stability and reliability. For example, linear stripe lasers, typically including either buried heterostructure or ridge waveguide lateral index guiding, have attained emitted powers of up to 700 mW in reports dating to 1992. However, these devices typically suffer from a high optical power density of 10-20 MW/cm2 at the diode""s output facet, which tends to degrade performance.
Some high-power, semiconductor laser diodes utilize a device structure with a light source (termed xe2x80x9cmaster oscillatorxe2x80x9d) and other components, all integrated onto a common semiconductor substrate. For example, a master oscillator power amplifier (MOPA) laser includes an oscillator and a high-gain optical amplifier that are monolithically integrated.
FIG. 1 shows an exemplary structure for a tapered-amplifier MOPA laser 100 of the prior art. MOPA laser 100 comprises single-mode laser diode oscillator 101, optional pre-amplifier 102, and optical power amplifier 103 that are formed on a common substrate 110. Single-mode laser diode oscillator 101 includes active region 120 and a gain region 121 that, along with the adjacent layers above and below them, form a transverse waveguide (i.e., a waveguide with direction parallel to the plane of the active region 120). However formed, MOPA laser 100 includes a light emitting region (e.g., active region 120) near a p-n-junction. Pump current applied to electrodes 111 and 112 greater than the lasing threshold current causes lasing (i.e., generation of amplified lightwaves) in active region 120 and gain region 121. MOPA laser 100 includes facets AR1 and AR2 that have anti-reflective coatings to minimize residual reflection of lightwaves within MOPA laser 100.
If a distributed Bragg reflector (DBR) laser is employed for the single-mode laser diode oscillator 101, gain region 121 is bounded by first- and second-order gratings 122 and 123. Gain region 121 may be formed by a lateral real refractive index waveguide material structure. Optional pre-amplifier 102 may be employed to optimize signal level and adjust beam shape of the lightwave produced by single-mode laser diode oscillator 101 that is subsequently applied to the following optical power amplifier 103. Pre-amplifier 102 typically includes a single-mode waveguide region that may be tapered. The single-mode waveguide region is formed from layers 115 adjacent to the active region 120, may be electrically isolated from single-mode laser diode oscillator 101, and is energized with pump current applied to electrodes 111 and 113.
Optical power amplifier 103 is coupled to pre-amplifier 102. Optical power amplifier 103 generally includes a transverse waveguide region about active region 120. In the transverse waveguide region, active region 120 is sandwiched between adjacent higher-bandgap, lower-refractive-index layers. Optical power amplifier 103 is electrically isolated from optical preamplifier 102 and is energized with pump current applied to electrodes 111 and 114.
A drawback of the MOPA laser structure of FIG. 1 is that the amplification of the beam emitted from the oscillator occurs when the beam passes through a relatively high-gain amplifier (e.g., optical power amplifier 103). The high-gain amplifier may have a typical single-pass gain in the neighborhood of 15 to 30 dB. In contrast, in solid-state lasing media supporting large, high-power optical modes (e.g., Nd:YAG rod external cavity lasers), the single-pass gain is relatively low (e.g., on the order of 0.1 dB per pass).
In a high-gain amplifier, a semiconductor region that supports multiple, propagating optical modes exhibits non-linearities associated with the optical amplification process. The non-linearities of the amplification process result from saturation of gain and cause beam distortions, including both those known as xe2x80x9cself-focusingxe2x80x9d which is related to the phenomenon known as xe2x80x9cfilamentationxe2x80x9d that tends to distort the wavefront of the propagating radiation in an uncontrolled fashion. Self-focusing and filamentation arise in large part and are related to the Kramers-Kronig relationship between imaginary and real parts of the refractive index in the amplifier regions of the semiconductor. Self-focusing exists in many semiconductor laser structures, and is particularly pronounced in those structures that support more than one waveguide mode under pumped-cavity conditions. Unstable resonator lasers and surface-emitting, distributed-feedback lasers similarly exhibit distortion from self-focusing.
FIG. 2a illustrates the broad-area gain section optical intensity profile for a high-gain, high non-linearity gain section affected by self-focusing and filamentation. As shown in FIG. 2a, a plot of optical intensity versus wavelength position indicates that the wavefront exhibits an irregular shape about the center position 75 xcexcm, and is thus long-term unstable. FIG. 2b illustrates the broad-area gain section optical intensity profile for a low-gain, low non-linearity gain section not affected by self-focusing. As shown in FIG. 2b, a plot of optical intensity versus wavelength position indicates that the wavefront exhibits a smooth roll-off shape about the center position 75 xcexcm, and is thus long-term stable.
Most laser diodes are edge emitting and are so called because the light beam emits from the cleaved edge of the processed laser diode semiconductor chip (e.g., through facet AR2 of FIG. 1). These types of laser diodes are commonly termed Fabry-Perot (FP) laser diodes since the laser diode cavity is similar to that of a conventional gas or solid state laser, but the cavity is formed inside the semiconductor laser diode chip itself. Mirrors may be formed by the cleaved edges of the chip, or one or both of the cleaved edges may be anti-reflection (AR) coated and external mirrors are added.
A vertical-cavity, surface-emitting laser (VCSEL), on the other hand, emits its beam from the top surface, and potentially the bottom surface, of the semiconductor chip. The cavity comprises a hundred or more layers of mirrors and active regions formed epitaxially on a bulk (inactive) substrate.
VCSEL devices exhibit the characteristics of low threshold current and low power when compared to other semiconductor laser diode devices that emit single-mode radiation. Lower lasing threshold and drive current results in lower electrical power requirements, potentially faster modulation, simpler drive circuitry, and reduced radio frequency interference (RFI) emission. VCSEL devices are also more tolerant of fluctuations in power supply drive. Directly controlling current for continuous operation is generally sufficient without requiring an optical feedback path, such as a feedback signal generated from a monitor photodiode mounted near one of the facets.
Although VCSEL devices that exhibit higher power are known in the prior art, these devices emit multi-mode radiation. In general, the broad cavity structure of VCSEL devices does not contain an alignment mechanism for lightwaves propagating through the cavity to produce single-mode radiation. VCSEL devices operated under external cavity, optically pumped conditions may provide higher power while maintaining single-mode emission, but these devices are relatively complex and require unwieldy external optical elements to support the single-mode emission. Such external components generally comprise specially designed mirrors to provide the necessary alignment mechanism.
The present invention relates to a master oscillator, vertical emission (MOVE) laser including an oscillator, coupling region, and vertical cavity amplifier region formed on a common substrate. The coupling region may include separately defined expansion and grating regions. The emitted single-mode radiation of the oscillator passes through the expansion region, which is a substantially passive region that provides spatial expansion of the propagating single-mode radiation wavefront with little or no gain. The expanded single-mode radiation from the expansion region passes through the grating region, which provides coupling of the relatively broad wavefront into the cavity of the vertical-cavity amplifier from the expansion region. The expansion and grating regions may be configured to reduce or eliminate reflection back toward the oscillator of single-mode radiation propagating within the vertical-cavity amplifier. The cavity of the vertical-cavity amplifier is relatively broad in cross-sectional area and relatively short in length when compared to the cavity of the oscillator. The vertical-cavity amplifier operates as a low-gain-per-pass amplifier for incident single-spatial-mode radiation generated by the oscillator. Thus, radiation of relatively low optical power emitted from the oscillator controls single-mode radiation of relatively high optical power emitted by the vertical-cavity amplifier.
In accordance with an exemplary embodiment of the present invention, an integrated semiconductor laser includes an oscillator having a cavity and capable of generating single-mode radiation; a vertical-cavity amplifier having a cavity broader than the oscillator cavity; and a coupling region having a narrow end coupled to the oscillator cavity and a wide end coupled to the vertical-cavity amplifier. When operating, single-mode radiation received from the oscillator is expanded by the coupling region and coupled into the vertical-cavity amplifier, and the vertical-cavity amplifier emits single-mode radiation at an optical power greater than that of the single-mode radiation generated by the oscillator.